1. Field of the Invention
The present invention relates to a drive circuit for a semiconductor device, and more particularly, it relates to a drive circuit for driving an insulated gate transistor.
2. Description of the Background Art
Devices using an insulated gate transistor such as an IGBT (insulated gate bipolar transistor) or a MOSFET (metal oxide field effect transistor) experience noises such as radiation noises or those resulting from a terminal voltage. Such noises have been handled by increased gate resistance in a drive circuit for driving these transistors. On the other hand, the increased gate resistance results in a longer switching time, thereby disadvantageously causing increased switching loss.
In response, a drive circuit for a semiconductor device including a plurality of gate resistors has been designed. By selecting these resistors, switching time can be shortened. A drive circuit for a semiconductor device introduced in Japanese Patent Application Laid-Open No. 2001-352748 (pp. 5–7 and FIGS. 1–4) may also be responsive in which a gate voltage is applied in more than one stage to an insulated gate transistor. These drive circuits are operative to suppress current surge and noise generation at the time of turn-on of an insulated gate transistor, and to reduce switching loss.
Japanese Patent Application Laid-Open No. 6-291631 (1994, pp. 3–7 and FIGS. 1–10) further introduces a drive circuit for a semiconductor device in which an emitter-collector voltage of a transistor is detected and a gate resistance varies according to the detected value. A gate voltage thereby increased or decreased causes slowdown in switching speed. Time variation of the gate voltage of an insulated gate transistor at the time of turn-on is moderated accordingly with no dependence on a current capacity of the transistor, whereby current surge and noise generation are suppressed.
In a drive circuit for a semiconductor device comprising a plurality of gate resistors, time-consuming replacement of the gate resistors necessitates temporary shutdown of the device. Further, even a drive circuit for a semiconductor device has such a configuration that allows selection of a plurality of gate resistors experiences difficulty in consecutive selection of the gate resistors. Only step-by-step selection is allowed. Still further, such a drive circuit requires gate resistors which correspond in number to selection stages, thus resulting in large-scale circuit configuration.
In the drive circuit for a semiconductor device introduced in Japanese Patent Application Laid-Open No. 2001-352748, a gate voltage which is not less than a threshold voltage of the insulated gate transistor is applied in more than one stage. This results in a long period for turning on the insulated gate transistor, thereby causing slowdown in switching as compared with other types of drive circuits.
The drive circuit for a semiconductor device introduced in Japanese Patent Application Laid-Open No. 6-291631 (1994) detects an emitter-collector voltage of the transistor to control a gate voltage, which control employs the same way as in the drive circuit comprising a plurality of gate resistors which are selected therebetween. That is, the drive circuit of Japanese Patent Application Laid-Open No. 6-291631 (1994) experiences difficulty in consecutively changing the gate resistance. Only step-by-step change of the gate resistance has been allowed therein.